Indium tin oxide (ITO) is commonly used as a transparent conductive layer in display devices, but it has a number of drawbacks. Thick coatings of ITO, which have low surface resistivities, have significantly reduced optical transmission and are not flexible. Bending the coating causes the ITO film to crack so reducing conductivity.
Many applications, such as flat panel displays, require inexpensive transparent conducting layers, but a bus bar is required to transport current over large area displays.
An alternative means of providing a substantially transparent conductor capable of transporting current over large areas is to use a patterned thin metallic conductor, which is also flexible.
One drawback to this approach is that for supplying closely packed devices, e.g. pixel elements of a larger display device, the use of such a common transparent front plane only provides a non-uniform field. This drawback can be improved by the addition of a second layer of a material of lower conductivity, e.g. a conducting polymer.
A common failing of conducting polymers is that they strongly absorb throughout the visible region, thereby damaging optical transmission.
Photographically generated silver conductive tracks are known in the prior art.
GB 0585035 describes a process for making conducting tracks, using a silver image formed by traditional photographic methods which is then put through an electroless-plating process. This may or may not then be followed by an electroplating step to create conductive tracks.
U.S. Pat. No. 3,223,525 describes a process for making conductive tracks using a silver image formed by traditional light exposure methods, in which the silver image is then enhanced by electroless-plating using a physical developer to form conductive tracks.
Silver meshes with continuous conducting polymer layers are also known in the prior art.
U.S. Pat. No. 5,354,613 describes the use of conductive polymers as a transparent conductive thin film, for use as an antistat in photographic products.
WO 2004/019345 and WO 2004/019666 describe the use of a non-continuous metal conductor in conjunction with a continuous conducting polymer layer which is flexible.
US 2004/0149962 describes the use of conductive polymers as transparent conductive layers within a non-uniform conductive metal entity and though this example is more flexible all conductive polymer molecules are significantly coloured compounds, which therefore reduces their optical transmission when coated.
US2005/0122034 describes the use of a layer containing transparent metal oxides in an organic material in conjunction with a layer containing a netlike structure comprising a thin metal line. Metal oxides generally have high refractive indices which as dispersed particles introduce scattering losses.
It is an aim of the invention to improve the electrical field uniformity in a non uniform conductive metal entity without reducing the optical transmission or limiting the flexibility.